Perpetual calendars have been devised in the past to provide time records of the days of the months of any year, as opposed to calendars that display the days of only one particular month or year. Such calendars typically include a number grid that is selectively positioned behind one or more windows through which the days of certain months are displayed. The number grid can be repositioned relative to the window or windows to display days in different months of different years. Two examples of perpetual calendars are disclosed in the following U.S. patents: U.S. Pat. No. 1,949,328 to Pinkerton; and U.S. Pat. No. 3,427,740 to Heskes.
The patent to Pinkerton discloses a perpetual calendar that is the size and shape of a business card, and is designed to be used as such. This calendar includes only one window through which the number grid can be viewed and therefore displays only one month at a time. The number grid is slid back and forth horizontally behind the window in an outer sleeve-like container, which includes the window on one side. Next to the window on the outside of the container are various characters and legends that must be deciphered to determine the month and year of the calendar month displayed in the window.
While the Pinkerton calendar is advantageous because of its small size, it has several disadvantages. First, it only displays one month at a time. Also, the month of the year being displayed is not clearly indicated on the container, but must be deciphered using a rather complex table that occupies more space on the device than the calendar window itself. In addition, the proper position of the number grid relative to the window is only determined after deciphering the complex table printed on the container. In short, the Pinkerton calendar is too complex for simple reference, because it requires time-consuming and difficult decoding steps to make use of the device. Another disadvantage is that because there is only one number grid provided, shorter months such as February appear to have more days than that particular month actually has due to the attempt at using a single number grid for all twelve months. Still another disadvantage is that there is nothing to hold the grid in one position within the container. The grid is free to slide in and out of the container, possibly resulting in its loss, and at least requiring repositioning every time a user refers to the Pinkerton calendar.
The patent to Heskes discloses a perpetual calendar that also includes a number grid in the form of a slide that is slid horizontally back and forth inside a sleeve. The sleeve includes seven display windows, each of which simultaneously corresponds to at least one month. Two of the windows may display up to three months at a time. The seven windows are shown arranged horizontally, side by side, in one embodiment and in three tiers in another embodiment. The sleeve also includes century and year slots to permit the selection of any one year calendar in any of a plurality of centuries. The slide includes an arrangement of sixty-one vertical columns of daily dates consisting of rows of dates running from 1 to 31. In any position within the sleeve, forty-nine of the date rows are displayed in the seven windows. The remaining rows are hidden behind sections of the sleeve between the windows. The slide also includes century and year indicators, which are visible in the century and year slots in the sleeve to indicate the calendar year being displayed.
The Heskes calendar is advantageous over the Pinkerton calendar because it required no deciphering of a table to determine which month and year is being displayed. However, the Heskes calendar is still disadvantageous because it displays twelve months in seven windows. Using one window to display more than one month creates an unnatural appearing calendar, which is not as easily used as a conventional twelve month calendar. In addition, the months are arranged out of their natural sequential order among the seven windows. Another problem, which is also present in the Pinkerton calendar, is that the Heskes calendar assumes the user will know the number of days in each month. For example, the fourth month of the Heskes calendar displays the dates in the months of February, March, and November. However, all three of these months have a different number of days. Other problems with the Heskes calendar include the same as those with the Pinkerton calendar, such as the lack of any provision to hold the slide fixed in one spot within the sleeve.
Thus, there remains a need for a new and improved perpetual calendar that is adjustable to permit the days of the months of any year to be displayed, while at the same time having the appearance of a conventional single-year calendar to avoid the possibility of errors and confusion when reading the perpetual calendar.